"We developed an approach to find the location of trapped miners inside a collapsed mine, regardless of noise from the environment around the mine," said Sherif Hanafy, an adjunct associate professor of geology and geophysics at the University of Utah and first author of a study demonstrating the technique.
The method records "seismic ‘fingerprints' generated by a trapped miner banging on the mine wall, and uses those fingerprints to locate him. Each different location in the mine that is banged has a unique fingerprint," said Gerard Schuster, a professor of geology and geophysics at the University of Utah and the study's senior author.
"We hope to make it easier to find out if miners are alive after a collapse and, if they are alive, where they are located," he added. "It's not guaranteed to work every time, but looks promising from the tests we did. This is not rocket science; it's rock science."
The researchers and a number of Utah graduate students tested the system twice. One test was in a utility tunnel beneath the University of Utah campus. The other test was in much deeper tunnels in an abandoned copper mine near Tucson, Ariz.
"We got 100 percent accuracy," Hanafy said.
Schuster explained more testing is needed to make sure the method will work in deeper mines, such as coal mines, which can be a few thousand feet deep. He says that while the method was tested only in horizontal mines tunnels, it also should work in vertical shafts.
The new study was published in The Leading Edge, a journal of the Society of Exploration Geophysicists.
How It Works
The system would be installed in stages as a mine is excavated. Components include:
• Inside mine tunnels, "base stations" are built every 10 yards to every few hundred yards, depending on each tunnel's length. At each station, a 4-inch-by-4-inch iron plate is bolted to the wall, and a sledgehammer is placed near each plate.
• On the surface, cables are strung along the ground above each tunnel or shaft, and "geophones" are spaced at regular intervals along the cables. Geophones listen for seismic waves created when miners use the sledgehammer to bang on an iron plate.
• Once the system is installed, and as the mine expands and base stations are added, each base station is calibrated, meaning its plate is whacked and the seismic waves are recorded by the geophones overhead. Each base station has a distinct seismic wave fingerprint. So if miners are trapped and bang the metal plate at the nearest base station, the resulting seismic recording will allow rescuers to determine precisely which base station plate was thumped, and thus where the miners are located.
Listening stations would record the seismic wave pattern from each geophone. A computer would compare the collective pattern with the calibration seismograph recordings collected prior to the collapse. A match identifies the base station or stations where survivors have gathered and walloped the iron plate.
The system would include perhaps 100 geophones and 100 base stations, and cost about $100,000 for a typical mine – an amount Schuster considers inexpensive.
"It's like having a fire extinguisher on every floor. How much does that cost?"
Schuster says the system could be expanded – at about double the cost – to allow two-way communications, instead of just signals from trapped miners to rescuers on the surface. Two-way communication would require a computer and geophone at each underground base station to pick up signals from people on the surface.
Hanafy says if miners were unable to reach the nearest base station, simply banging on a mine wall with a rock should produce a "fingerprint" that identifies the nearest base station.
The Utah scientists tested the method in an Arizona copper mine, where they placed 25 base stations 1.6 feet apart in a 100-foot-deep tunnel, and another 25 base stations 2.5 feet apart in an underlying 150-foot-deep tunnel. On the surface, 120 geophones were set up along a 200-foot-long line running above the two tunnels. Every bang on a base station was accurately located.
Schuster said that to "simulate battlefield conditions" at a working mine, a computer was used to simulate white noise that drowned out the real seismic signals by a 2,000-to-1 ratio. He says the seismic signature of a bang on a base station plate still could be distinguished.
"It's like at a cocktail party you have 2,000 people talking at the same time in different conversations, and somehow you can home in on one conversation," Schuster said.